Literally the only time we couldn't get a slot when we wanted it was at Casey's right before fireworks. It was an hour out. Never had a problem otherwise, and we visit the park 100+ times a year. I don't plan ahead that far to order hours in advance. It's usually 15 minutes before as we're walking to the restaurant.
To add, you don't have to mobile order. If you want to eat now, just walk in and order at the register. If the timeslot is 30 minutes away and you need some AC now (Florida...), wander in and order in person. Sometimes we mobile order and sometimes we just get in line. Mobile ordering is not always fastest. Captain Cooks, for example, is nearly always faster in person. Same with Gasparilla.
Once you click your timeslot and submit your order, you can always reschedule it or cancel it altogether. You're not charged until you click "I'm here."